Polymer polyols are commonly used to produce polyurethane foam having improved properties. The presence of the solid polymer particles dispersed in the polyol facilitates cell opening and provides increased loadbearing properties to the resulting polyurethane foam. These foam property improvements are a function of the solid content of the polymer polyol, the polymer particle size, the polymer particle size distribution, and the polymer particle morphology.
U.S. Pat. No. 4,148,840 to Shah teaches a continuous process for improving the stability of polymer polyols by adding a minor portion of a preformed polymer polyol to a base polyol together with the monomers and initiator. Specifically, the preformed polymer polyol minor portion consists of from about 45 to about 5 wt %, most preferably from about 40 to about 10 wt % of the total batch. The preformed polymer polyol portion taught by., Shah is preferably prepared in a high molecular weight polyol or dispersant polyol. No mention of particle size and particle size distribution of the dispersed solid polymer particles is made.
A two step process for preparing polymer polyols having a broad polymer particle size distribution via a semi-batch process is taught in U.S. Pat. No. 5,223,570. The two step process requires preparing an intermediate polymer polyol having a solids content of up to 30% in a continuous process followed by increasing the solids content to above 30% in a semi-batch process. The advantage of this two step process is its ability to produce polymer polyols without wildly fluctuating viscosities. The obvious disadvantage of this method is it requires two different types of reactors.
European Patent Application No. EP 0510533A2 teaches that polymer polyols having improved stability can be prepared with the use of a high molecular weight monol containing terminal unsaturation. The use of a seed dispersion is discussed; however, the seed is a polymer polyol made by polymerizing monomers in the monol stabilized polyol. Moreover, the seed particles have an average particle size of 0.05 to 0.15.mu.. Polymer polyols with broad particle size distributions are not taught.
Polymer polyols prepared from preformed stabilizers are disclosed in U.S. Pat. No. 5,196,476. The preformed stabilizer is prepared by polymerizing ethylenically unsaturated monomers in a polyol having reactive unsaturation and a liquid diluent.